1. Field of the Invention
The invention relates to semiconductor device fabrication and more particularly to an apparatus for detecting heights of particles on a photoresist-coated optical glass in the steps of photolithography.
2. Description of Related Art
As shown in FIG. 1, conventional photolithography includes several steps in sequence. For example, after drying and cooling a photoresist-coated optical glass (glass in short), exposure and developing steps are performed on the glass. It is known that top of the glass may have defects (e.g., particles). It is also known that heights of particles on the glass are required to be less than a set value, for example, 100 μm. Otherwise, an expensive mask subsequently placed on the glass may be damaged by the particles and this is undesired. Thus, an apparatus for detecting heights of particles on a glass in the steps of photolithography is required.
A conventional apparatus for detecting heights of particles on a glass in the steps of photolithography is shown in FIG. 2. The apparatus is a gantry type and comprises a transmitter A, a receiver B, and a bridge C having two ends secured to the transmitter A and receiver B respectively. A rectangular glass D is placed on a stage E which is mounted between the transmitter A and receiver B. The transmitter A and the receiver move toward the same direction in synchronous in a defect detection operation. The transmitter A can emit laser beam (indicated by dotted lines) toward the receiver B and the laser beam transversely passes through a space about 150-200 μm above the glass D. No signal is generated at the receiver B if there is no particle having a height less than 150 μm. To the contrary, a signal is generated at the receiver 1B if the laser beam is blocked by a particle F having a height equal to or more than 150 μm. The signal is converted into a representation of unacceptable defect after a processing.
The synchronous movement of the transmitter A and receiver B is the advantage of the conventional apparatus. However, the conventional apparatus also has a disadvantage of having a fixed distance between the transmitter A and receiver B. That is, an adjustment of the distance between the transmitter A and receiver B is impossible. This greatly decreases the application of the conventional apparatus because widths of the glasses F produced by different companies are not the same. In short, the conventional apparatus is applicable to a limited number of applications. Further, particles having a height in the range of 100-150 μm cannot be detected. Unfortunately, the particles can still damage a mask if the mask is placed on the glass in a subsequent manufacturing step. Furthermore, a tact time is about 30 seconds and is unacceptable. Thus, the need for improvement still exists.